The pharmaceutical preparations for oral administration according to the prior art begin to disintegrate or dissolve before the arrival at the large intestine. Therefore, it has been a practice in the prior art to employ an intravenous, intranasal or rectal administration method in the administration of an active substance which is expected to exhibit a local action on the large intestine or is decomposed by the lyase present in the small intestine.
However, an oral pharmaceutical preparation has the following advantages. Namely, if an oral pharmaceutical preparation releases a pharmacologically active substance selectively in the large intestine, the concentration of the active substance at the release site can be increased to give a more effective local action than that of the preparation according to the prior art. Further, if a pharmacologically active protein or peptide which is decomposed in the small intestine loses its activity and is released selectively in the large intestine, the protein or peptide will be able to be absorbed in the large intestine wherein the protease activity is poor. Owing to the above advantages, it is apparent that an oral pharmaceutical preparation is a dosage form far more useful than injection or suppository.
For the reason described above, many attempts have recently been made to develop oral pharmaceutical preparations targeting the large intestine.
For example, there have been reported an oral preparation targeting the large intestine, which is prepared by combining a polymer soluble only at a pH of 5.5 or above with an insoluble polymer (EP40590); a solid oral preparation coated with a suitable amount of an anionic polymer soluble at a pH of 7.0 or above (trade name: Eudragit S, a product of ROHM) (WO83/00435); an oral preparation coated with a mixture comprising an anionic polymer soluble at a pH of 7.0 or above (trade name: Eudragit S or L, a product of ROHM) and a methacrylate copolymer difficultly soluble in water (trade name: Eudragit RS, a product of ROHM) at a suitable ratio (EP225189); an osmotic-pump preparation coated with an enteric polymer (BE903502); a colon-reaching oral preparation covered with an inner coat soluble at a pH of 7.0 or above, an intermediate coat made of a gelatinized polymer, and a acid-resistant outer coat soluble at a pH of 5.5 or above (Japanese Patent Publication No. 501411/1992); and so forth.
All of these preparations are characterized in that a drug is released by a pH increase in the intestinal tract, and they are each prepared by applying a proper amount of an anionic polymer or by adding or applying an insoluble polymer to prevent the drug from being released during its residence in the small intestine.
Meanwhile, a recent study on humans (D. F. Evans, Gut, 1988, vol.29, p.1035) reported that the pH of the intermediate and lower parts of the small intestine ranges from 7.4 to 7.5 (standard deviation: 0.4 to 0.5), while the average pH rapidly drops to 6.4 (standard deviation: 0.4) in the upper part of the large intestine and gradually increases to reach 7.0 (standard deviation: 0.7) in the lower part of the large intestine.
However, all of the above oral preparations releasable in the large intestine according to the prior art are coated with a polymer soluble or swelling at a pH of 7.0 or above, on the assumption that the pH in the colon or large intestine is 7.0 or above. The inventors of the present invention have also made intensive studies on this respect and have found that the pH of cecal contents of a rat or rabbit is below the range of 6.4 to 5.5 owing to the organic acids generated by enteric bacteria. This result agrees with that disclosed in the above literature on humans. Accordingly, the inventors of the present invention have started studies to develop an oral pharmaceutical preparation from which a drug is released in the lower digestive tract (large intestine) by a pH change to below the range of 7.0 to 5.5.
Further, WO90/13286 discloses an oral preparation comprising a core containing a basis and a filler, a first layer which covers the core and is composed of an enteric component, a second layer which covers the first layer and is composed of a nonenteric component, and a third layer which covers the second layer and is composed of an enteric component. In this oral preparation, the second layer, which is composed of a cationic polymer, is covered with the third layer to regulate the release of a drug, so that it is very difficult to regulate the preparation so as to release a drug specifically in the large intestine. Further, since the core always contains an acidic substance, the second layer is liable to be dissolved in the small intestine to release a drug prior to the arrival at the large intestine.
The coat according to the prior art has such a property that the dissolution or swelling thereof proceeds before the arrival of the preparation at the large intestine. Therefore, the preparation of the prior art has a disadvantage that when the actual residence time in the small intestine, which has a great physiological influence, is shorter than the one assumed in the dosage form design, the preparation is discharged as such, while when the actual residence time is longer than the assumed one, the release of a drug has been completed before the arrival at the target site.